1. Field of Invention
The present invention relates to an apparatus for controlling ribbon tension and a method for controlling the same. More particularly, the present invention relates to an apparatus for controlling ribbon tension in a thermal printer and a method for controlling the same.
2. Description of Related Art
In a thermal printer, a ribbon is used as a medium for transferring image. To obtain high print quality, a uniform ribbon tension should be maintained during the printing process. If ribbon tension is too low in the printing process, ribbon tension cannot lead the ribbon forward; the ribbon may even become slack, resulting in a torn ribbon by mechanism. If ribbon tension is too high, the ribbon may wrinkle and lower the print quality, and the ribbon may be deformed.
In general, the ribbon is unwound from a supply reel and collected on a take-up reel. The take-up reel in the thermal printer is usually driven by a step motor or a direct current motor, which provides a constant torque. The supply reel is not connected to a motor; thus, ribbon tension on the supply reel is only maintained by the frictional force between the supply reel and the ribbon. During the printing process, for a constant torque provided by the motor connected to the take-up reel, ribbon tension is governed by the equation: EQU tension=torque/radius of take-up reel
Consequently, ribbon tension is varied during the operation of the thermal printer by a change in the radius of the take-up reel.
Furthermore, to improve the print quality, the thermal print head is usually directly in contact with the ribbon during the printing process; thus, the frictional force between the thermal print head and the ribbon and the frictional force between the ribbon and the paper are both tremendously increased. Ribbon tension is also needed to be boosted up.
In addition, different functions are performed by the ribbon during the operation of the thermal printer, and these functions are achieved by controlling ribbon tension and the direction in which the ribbon moves. For example, in the preparation process, the ribbon should lead the paper into an engagement position between the capstan roller and the pinch roller. During the printing process, the ribbon should overcome the friction between the ribbon and the thermal print head and the friction between the ribbon and the paper. After a certain color panel is printed, the ribbon should run reversely to conduct the paper back into the back-feed path smoothly.
In U.S. Pat. No. 5,138,335, a spring is directly in contact with the supply reel and the take-up reel to control ribbon tension. In U.S. Pat. No. 5,529,410, ribbon tension is controlled by using pulse width of the motor connected to the take-up reel; thus, ribbon tension on the take-up reel is maintained uniform during the printing process. However, the structures in these patents mentioned above are complex, and their operations obviously do not satisfy the extra functions performed by nowaday thermal printing system.